Surfactant droplet evaporation and deposition patterns on waxy leaf surface

Abstract

Leaf surfaces are often altered when attacked by various insects and diseases. To evaluate the effect of droplet deposition on pesticide application efficiency, evaporation and deposit pattern formation of single droplets deposited at various locations on waxy leaves were investigated under controlled conditions. The locations on leaves included the intervenial area, midrib and secondary vein on both adaxial and abaxial surfaces. Tests were conducted with 300 and 600 µm droplets containing water and a nonionic surfactant. The ambient temperature was 25oC and relative humidity was 60%. Evaporation time and wetted area of droplets varied with where droplets deposited on leaf surfaces. The difference in evaporation time of 300 µm droplets without the surfactant on the intervenial area, midrib and secondary vein of adaxial surface was 30% while the difference in wetted area was 39%. The wetted area was significantly larger on the adaxial surfaces than on the abaxial surface but the evaporation time between both surfaces was not significantly different. For the whole leaf, the average evaporation time of 300 µm droplets was decreased by 44% and the average wetted area was increased by 202% when 0.25% nonionic surfactant was added into the spray solution. The total mean evaporation time increased 279% and the total mean wetted area increased 166% without the surfactant, 452% and 229% with the surfactant when the droplet diameter was changed from 300 to 600 µm. The largest spread area of droplets was appeared on the midrib of adaxial surface with the surfactant. The 300 µm droplets had longer evaporation time per droplet volume and greater wetted area per droplet volume than the 600 µm droplets. Increased pesticide application efficiency could be achieved with smaller droplets containing the surfactant to enhance the droplet spreading at different positions on waxy leaves.